In accordance with social demands to energy saving in recent years, silica is blended and used so frequently as a filler which causes a low heat generating property of a rubber composition for tires to be consistent with a gripping property of a tire on a wet road surface for the purpose of saving fuel consumption of automobiles.
Silica used tends to cause coagulation of particles themselves by virtue of a hydrogen bond of a silanol group which is a surface functional group, and in order to improve a dispersibility of silica in a rubber, a kneading time has to be elongated. Also, silica has the defect that a Mooney viscosity of a rubber composition is raised due to an unsatisfactory dispersibility of silica in a rubber and that the rubber composition is inferior in a processability such as extrusion. Further, the surface of silica is acidic, and therefore it involves the defect that it adsorbs thereon basic substances which are used as a vulcanization accelerator to prevent the rubber composition from being sufficiently vulcanized and that the storage modulus is not enhanced. Accordingly, silica-blended rubber compositions have so far been desired to be improved in a processability and the like.
There have so far been known as a technology for improving a processability and the like in a silica-blended rubber composition prepared by using glycerin fatty acid esters, for example:    1) a rubber composition improved in an electrostatic property which is prepared by blending 100 parts by weight of a rubber containing 90 parts by weight or more of a diene base rubber with 30 to 120 parts by weight of a filler containing 40% by weight or more of a white filler and 0.2 to 8 parts by weight of a nonionic surfactant (refer to, for example, a patent document 1, and    2) a rubber composition for a tire tread containing at least one polymer selected from the group of diene base rubbers, and 5 to 100 parts by weight of fine powder non-precipitated silicic acid, 0 to 80 parts by weight of carbon black, and 5 to 80 parts by weight of at least one non-aromatic viscosity-reducing substance each based on 100 parts by weight of a rubber contained in the rubber composition,wherein the non-aromatic viscosity-reducing substance described above is at least one substance selected from the group consisting of glycerin monostearate, sorbitan monostearate, sorbitan monooleate, and trimethylolpropane(2-ethyl-2-hydroxymethyl-1,3-propanediol) (refer to, for example, a patent document 2).
On the other hand, known as a technology for improving a dispersibility of silica in a rubber, a processability and the like in a silica-blended rubber composition prepared by using compounds other than the glycerin fatty acid esters described above are, for example:    3) (A) a composition prepared by blending 15 to 85 parts by weight of silica based on 100 parts by mass of a rubber component containing a natural rubber and/or a diene base synthetic rubber, and a specific tertiary amine compound such as dimethylalkylamine and the like in a proportion of 1 to 15% by weight based on an amount of the silica, and a pneumatic tire prepared by using the above composition for a tire tread (refer to, for example, a patent document 3), and    4) a rubber composition for a tire tread prepared by blending 100 parts by weight of a rubber composition containing a natural rubber and/or a diene base rubber with a white filler and at least one specific monoalkanolamide, and a tire prepared by using the above composition (refer to, for example, a patent document 4).
It is described in the patent document 1 out of the patent documents 1 to 4 that glycerin fatty acid monoester is blended in one of the examples to obtain an effect of preventing electrification which is likely to be caused in blending silica and which is different from that of the present invention, but a viscosity-reducing effect is neither described nor suggested therein.
Also, a technology close to the present invention is disclosed in the patent documents 2 described above, and it is described therein that glycerin fatty acid monoester is blended to obtain a viscosity-reducing effect in blending silica. However, shrink (deterioration of a processability caused by shrinkage) which is likely to be caused in blending a silica-blended rubber composition with the glycerin fatty acid monoester is neither described nor suggested therein.
Further, the rubber compositions prepared in the patent documents 3 and 4 described above are improved in a dispersibility of silica in a rubber and a heat generating property to such an extent as has not so far been observed, but it is reduced slightly in a processability due to shrinkage. Also, the problem that the scorch time is expedite to cause rubber yellowing is brought about in the patent document 3.